1. Field of the Invention: This invention relates to the propulsion of marine vessels and is particularly concerned with a hydraulic system for providing thrust for ships and similar vessels.
2. Description of the Prior Art: Hydraulic propulsion systems in which seawater is discharged from an underwater passageway in a ship's hull to generate thrust have been used for maneuvering ships, barges and similar marine vessels. Such systems generally depend on an internal propeller to pull water into one end of a passageway and discharge it at low velocity from the other end. The volume of water which must be moved through the passageway is generally large and hence such systems are expensive to install and operate. It has been suggested that jet eductors might be used in lieu of the propellers. By pumping water through an independent line to an internal nozzle mounted near the centerline of the passageway adjacent a downstream venturi section, a differential pressure sufficiently high to induce the flow of additional water into the intake end of the passageway can be created. This results in higher mass flow and increases the thrust developed by the system. By using a rotatable nozzle and providing a venturi section on each end of the passageway, the thrust can be applied in either direction. If a passageway having two or more branches is used and provisions are made for closing off one or more branches when not in use, greater directional control and better maneuverability of the vessel can be obtained.
Although eductor systems of the type outlined above offer interesting possibilities, efforts to develop a practical system have not been encouraging. Laboratory studies and model tests have shown that the total thrust developed by such systems tends to be low. The additional water which flows into the intake end of the passageway in response to the differential pressure created in the venturi section normally gives an increase in thrust of only about 5% about that provided by water discharged through the internal nozzle alone. The total thrust can be increased by pumping more water through the nozzle but this is expensive and does not significantly improve the overall efficiency of the system. As a result, the economics of using educter-type hydraulic propulsion systems have been marginal at best.